Semiconductor memory element, semiconductor device and control method thereof

ABSTRACT

In a semiconductor flash memory required to have high reliability, injection and extraction of electrons must be performed through an oxide film obtained by directly oxidizing a silicon substrate. Accordingly, the voltage to be used is a large voltage ranging from positive to negative one. In contrast, by storing charges in a plurality of dispersed regions, high reliability is achieved. Based on the high reliability, transfer of electrons is permitted through not only the oxide film obtained by directly thermally oxidizing a high reliability silicon substrate but also another oxide film deposited by CVD, or the like. In consequence, a device is controlled by electric potentials of the same polarity upon writing of data and upon erasing of data.

BACKGROUND OF THE INVENTION

The present invention relates to a semiconductor memory element and asemiconductor device.

In recent years, flash memories which are introduced into a large numberof apparatuses as the ones for storing programs or for storing data. Theproblem encountered with the flash memories is the price thereof. Theprice per capacity thereof is several or more fold higher as comparedwith other media such as hard disks, magneto-optic disks, and DVDs,resulting in a demand for cost reduction. The cost reduction can beachieved most effectively by a decrease in chip area. In contrast, therehas been adopted in the prior art an approach of reducing the area ofthe memory cell. This is implemented by physically reducing the memorycell size due to miniaturization. One example of the memory cell sizereduction due to miniaturization is described in H. Miwa et al. “A 140mm² 64 Mb AND Flash Memory with A 0.4 μm Technology” IEEE, InternationalSolid-State Circuit Conference 1996, p34-35 (1996). Alternatively, theso-called multi-level technology has come into actual use, which enablesevery memory cell to store two bits of information, thereby toeffectually reduce the memory cell area per bit, or other approacheshave been made. The prior art example of the multi-level memory isdescribed in T. Jung et al., “A 3.3V 128 Mb Multi-Level NAND FlashMemory for Mass Storage Applications” IEEE International Solid-StateCircuit Conference 1996, p32-33 (1996).

SUMMARY OF THE INVENTION

For ensuring the reliability, in a flash memory, scaling cannot beperformed in the direction of thickness. Therefore, it is not possibleto set the operation voltage at a lower level. Similarly, for ensuringthe reliability, electron transfer must be performed through an oxidefilm formed by directly thermally oxidizing a silicon substrate. Theoxide film is less susceptible to charge leakage. Accordingly, use oflarge positive and negative voltages is unavoidable. For this reason,the peripheral circuit, particularly, the power source occupies largearea. As a result, the proportion of the area of the memory cells isreduced, leaving a problem that the chip area cannot be reduced eventhrough miniaturization. The increase in cost due to a reduction inproportion of the memory cell area presents a large problem for aflash-embeded logic circuit for incorporation into an apparatus, or thelike.

An object of the present invention is to provide a memory elementconfiguration whereby the required voltages are few in kind, and thevoltage is low, while ensuring the reliability. By using the memoryelement, it becomes possible to simplify the configurations of theperipheral circuits of a semiconductor memory device, and thereby toreduce the chip area. Namely, it becomes possible to provide a methodfor implementing a low cost semiconductor memory device.

The present invention is characterized in the following respects.Charges are not stored in a single region in a memory cell as in theprior art, but stored in a plurality of dispersed regions. Inconsequence, high reliability is implemented. The operation mode issimplified by performing electron transfer through not only the oxidefilm obtained by directly thermally oxidizing a high reliability siliconsubstrate but also another oxide film obtained from deposition by CVD(Chemical Vapor Deposition), or the like. As a result, the costreduction of the semiconductor memory device is achieved.

More specifically, a semiconductor device in accordance with a typicalembodiment of the present invention is mainly made up of semiconductormemory elements, each of which has:

a source region,

a drain region,

a channel region made of a semiconductor,

the source region and the drain region being connected by the channelregion,

a gate electrode made of a metal or a semiconductor for controlling theelectric potential of the channel region, and

a plurality of charge storage regions in the vicinity of the channelregion,

wherein the electric potential to be applied to the gate electrode uponwriting of data and the electric potential to be applied to the gateelectrode upon erasing of data have the same polarity.

Other means, objects, and features of the present invention will becomeapparent from the following embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the configuration of the cross section of a semiconductormemory element of Example 1;

FIG. 2 is a representation on a circuit diagram corresponding to thesemiconductor memory element of Example 1;

FIG. 3 is a cross sectional diagram of a semiconductor memory element ofExample 2;

FIG. 4 is a representation on a circuit diagram corresponding to thesemiconductor memory element of Example 2;

FIG. 5 is an equivalent circuit diagram of a semiconductor memory deviceof Example 3;

FIG. 6 is an equivalent circuit diagram of a semiconductor memory deviceof Example 4;

FIG. 7 is a layout diagram of the semiconductor memory device of Example4;

FIG. 8 is a diagram showing the configuration of the cross section of asemiconductor memory device of Example 5;

FIG. 9 is an equivalent circuit diagram of the semiconductor memorydevice of Example 5;

FIG. 10 shows an equivalent circuit of a semiconductor memory device ofExample 6;

FIG. 11 is a cross sectional diagram of the semiconductor memory deviceof Example 6;

FIG. 12 shows an equivalent circuit of a semiconductor memory device ofExample 7; and

FIG. 13 is an isometric cross sectional diagram of the semiconductormemory device of Example 7.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1

Below, a semiconductor element and a semiconductor device will bedescribed by way of specific examples of the present invention.

FIG. 1 shows the configuration of the cross section of a memory elementin accordance with this example.

The memory element has a triple-well structure in which on a P-typesilicon substrate (A1), an n-type well region (A2) is disposed, and aP-type well region (A3) is further disposed therein. There are N-typesource region (A4) and drain region (A5) in the P-type well (A3). Aplurality of microcrystal grains (AB) with a mean size of 10 nm ofsilicon, serving as charge storage regions, are arranged on a channel(A6) via a 6 nm-thick insulation film (A7) made of SiO₂. There isdisposed a gate electrode (A9) made of N-type polysilicon forcontrolling the electric potentials of the channel and the chargestorage regions. The region between the silicon microcrystal grains (A8)and the gate electrode (A9) are constituted by an insulation film (A10)of a so-called ONO structure in which a 3 nm-thick SiO₂ film, 6 nm-thickSi₃N₄ film, and a 3 nm-thick SiO₂ film are stacked in this order fromthe lowest. Alternatively, it is also possible that the region betweenthe silicon microcrystal grains (A8) and the gate electrode (A9) isconstituted not by an insulation film of the ONO structure but by a 9nm-thick insulation film made of SiO₂.

FIG. 2 shows a representation in the circuit diagram corresponding toFIG. 1. The elements are respectively assigned their correspondingnumbers: gate electrode (A9), source (A4), drain (A5), and chargestorage region (A8). Incidentally, the triple-well structure isdemonstrated in FIG. 1, but it is omitted for avoiding the complicationof the diagrams in other examples.

Then, a manufacturing process of this example will be described. Afterforming an isolation region and the triple-well structure (A1), (A2),and (A3), B (boron) ion implantation for controlling the thresholdvoltage is performed on a memory cell formation region on the P well(A3). The substrate surface is oxidized to form the 6 nm-thick SiO₂ film(A7). Subsequently, silicon microcrystal grains are formed by CVD(Chemical Vapor Deposition). In a prototype, the grains were formed witha mean grain size of 10 nm and a density of 5×10¹¹ cm ⁻², On the siliconmicrocrystal grains (A8), an interlayer insulation film (A10) of the ONOstructure in which a 3 nm-thick SiO₂ film, 6 nm-thick Si₃N₄ film, and a3 nm-thick SiO₂ film are stacked in this order from the lowest isformed. Thereafter, N-type polysilicon for forming the gate electrode(A9) is deposited thereon, and a SiO₂ film is further deposited. Byusing a resist as a mask, the SiO₂ film, the polysilicon film, the ONOfilm, the silicon microcrystal grains, and the SiO₂ film aresuccessively etched. In this process, the gate electrode (A9) is formed.By using the gate electrode (A9) as a mask, As (arsenic) ions areimplanted thereinto, followed by activation annealing to form the sourceregion (A4) and the drain region (A5). Further, interlayer filmdeposition and planarization are performed, and then, a contact step anda wiring step are performed.

Then, the operation of this example will be described.

First, the write operation will be described. Herein, the state in whicha large amount of charges are injected in the charge storage regions(A8) is set to correspond to data “1”, while the state in which lessamount of charges are injected therein is set to correspond to data “0”.

Writing of the data “1” is performed in the following manner. Namely, 0V is applied to the source region (A4), a positive potential (ex., 5 V)to the drain region (A5), and a positive voltage pulse (ex., 5 V) to thegate electrode (A9). In consequence, channel hot electrons aregenerated, so that electrons are injected into the charge storageregions (A8). The following alternative is also possible. Namely, 0 V isapplied to the source region (A4) and the drain region (A5), and apositive potential (ex., 18 V) is applied to the gate electrode (A9). Inconsequence, electrons are allowed to tunnel through the insulation film(A7), and to be injected into the charge storage regions. In this case,a larger voltage is required than with the charge injection utilizinghot electrons, and hence, unfavorably, the configurations of theperipheral circuits become complicated.

Writing of the data “0” is performed by extracting electrons from thecharge storage regions (A8) to the gate electrode (A9). Specifically, 0V is applied to the source region (A4) and the drain region (A5), and apositive potential pulse (ex., 10 V) is applied to the gate electrode(A9). In consequence, electrons are allowed to tunnel through theinsulation film (A7), and to be extracted to the gate electrode (A9).Alternatively, writing of the data “0” can also be accomplished in thefollowing manner as with a conventional flash memory. Namely, a negativevoltage pulse (ex., −10 V) is applied to the gate electrode. Inconsequence, electrons are extracted from the charge storage regions,allowed to tunnel through the insulation film (A7), and to be attractedto the substrate. However, in this case, a negative voltage pulse mustbe utilized, entailing a demerit that the peripheral circuits becomecomplicated. Incidentally, in this example, writing of the data “0” issubstantially identical to erasing of the data.

A large number of operations of rewriting data equal to a large numberof operations of applying a voltage stress. The accumulation of thevoltage stresses causes the deterioration of the insulation film made ofSiO₂, so that electrons become more likely to leak even at a lowelectric field. The degree of the degradation is more severe for theSiO₂ film formed by CVD than for the SiO₂ film formed by oxidizing thesubstrate. Therefore, with a conventional flash memory, a voltage stresscan be applied only to an insulation film made of SiO₂ formed byoxidizing the substrate for rewriting data in order to store chargeswith stability. Namely, electron transfer must be restricted between thesubstrate and the charge storage region for ensuring the reliability.

In contrast, in this example, the charge storage regions are made up ofa plurality of silicon microcrystal grains. Therefore, the chargesstored in the silicon microcrystal grains existing on leak paths are theonly ones that leak even after electrons have become susceptible toleakage at a low electric field. Most other silicon microcrystal grainsare capable of holding charges with stability, and hence the wholeelement has a good charge retention characteristic. Therefore, even if avoltage stress is applied to the SiO₂ film formed by CVD susceptible toa voltage stress, in this example, it is possible to ensure the chargeretention characteristic comparable to that of a conventional flashmemory. Namely, electron transfer is possible not only between thesubstrate and the charge storage regions but also between the chargestorage regions and the gate electrode.

Then, the read operation will be described. For example, 2 V is appliedto the drain region, 0 V is applied to the source region, and a readpulse of 2 V is applied to the gate electrode (A9). The thresholdvoltage differs according to whether the amount of charges injected intothe charge storage regions (A8) is large or small. Accordingly, thedrain current when the data “0” is written is larger than the draincurrent when the data “1” is written. As a result, it is possible todistinguish between the data “0” and the data “1”. Alternatively,reading can also be accomplished by interchanging the voltagerelationship between the drain region and the source region like thefollowing relationship: 0 V for the drain region, 2 V for the sourceregion, and 2 V for the gate electrode.

The voltage relationships used for writing and reading of the data “1”and the data “0” in this example are summarized in Table 1.

TABLE 1 Source voltage Drain voltage Gate voltage Writing of “1” 0 V 5 V5 V Writing of “0” 0 V 0 V 10 V  (Data erasure) Reading 0 V 2 V 2 V

With a conventional flash memory, writing of the data “0” is performedin the following manner. Namely, a negative potential is applied to thegate electrode. In consequence, electrons are allowed to tunnel throughthe insulation film (A7), and to be extracted into the substrate. Theelectric potential applied to the gate is large because electrons areextracted into the substrate. Further, it is opposite in polarity to theelectric potential used for writing of the data “1”, and hence a powergeneration circuit becomes complicated, incurring not only an increasein chip size, but also an increase in cost.

In this example, writing and reading of either of the data “1” and thedata “0” can be both accomplished only by applying voltages with thesame polarity and almost the same magnitude. Therefore, the powergeneration circuit becomes simple. As a result, it becomes possible tolargely reduce the area of the peripheral circuits.

EXAMPLE 2

FIG. 3 shows the configuration of the cross section of a memory elementin accordance with a second example in the present invention.

There are N-type source region (A12) and drain region (A13) disposed ina P-type well (A11). A plurality of microcrystal grains (A17) with amean size of 10 nm of silicon, serving as charge storage regions, arearranged on channels (A14) and (A15) via a 5 nm-thick insulation film(A16). There is disposed a first gate electrode (A18) made of N-typepolysilicon for controlling the electric potentials of a part of thechannel (A15) and the silicon microcrystal grains (A17) The regionbetween the silicon microcrystal grains (A17) and the first gateelectrode (A18) are constituted by an insulation film (A19) of aso-called ONO structure in which a 3 nm-thick SiO₂ film, a 6 nm-thickSi₃N₄ film, and a 3 nm-thick SiO₂ film are stacked in this order fromthe lowest. Further, there is a second gate electrode (A20) forcontrolling the electric potential of a part of the channel region(A14).

FIG. 4 shows a representation in the circuit diagram corresponding toFIG. 3. The elements are respectively assigned their correspondingnumbers: first gate electrode (A18), second gate electrode (A20) sourceregion (A12), drain region (A13), and charge storage regions (A17) madeof silicon microcrystal grains.

Then, the operation of this example will be described. In this example,hot electron injection into the charge storage regions (A17) isperformed with high efficiency by using the second gate electrode (A20)as an auxiliary electrode.

First, the write operation will be described. The voltage applied to thedrain region (A13) is set according to the data to be written. Herein,the state in which a large amount of charges are injected therein is setto correspond to the data “1”, while the state in which less amount ofcharges are injected therein is set to correspond to the data “0”. Forwriting of the data “1”, the drain voltage is set so as to cause anelectric field enough for hot electron generation (ex., it is set to be5 V). To the source region (A12), 0 V is applied. The second gateelectrode (A20) is set at a prescribed voltage (ex., 2 V). A write pulsewith a high voltage (ex., 7 V), which is higher than that of the secondgate electrode (A20), is applied to the first gate electrode (A18). Atthis step, the resistance of the substrate surface (A14) under thesecond gate electrode (A20) is higher than the resistance of thesubstrate surface (A15) under the first gate electrode (A18).Accordingly, most of the voltage between the source and the drain isapplied to the underlying portion (A14) of the second gate electrode(A20). Further, also in the underlying portion (A14) of the second gateelectrode (A20), the side closer to the drain (A13) has a higherelectric potential, so that the effective gate voltage is low, resultingin a high resistance. For this reason, hot electrons are generated in alarger amount at the end of the underlying portion (A14) of the secondgate electrode (A20) closer to the drain (A13). The generated hotelectrons are accelerated in the direction of the charge storage regions(A17) under the electric field due to the first gate electrode (A18),causing injection. The hot electrons are injected intensively into asite (A21) underneath the first gate electrode (A18) and closer to thesecond gate electrode (A20). The current flowing between the source andthe drain at this step is smaller as compared with the configurationhaving no auxiliary gate because of the higher resistance of theunderlying portion (A14) of the second gate electrode (A20). Thisfavorably enables high efficiency injection, so that a small amount ofcurrent suffices. Therefore, it becomes possible to simplify theconfigurations of the peripheral circuits, particularly the powergeneration circuit.

Writing of the data “0” is performed in the following manner. Namely, apositive voltage is applied to the first gate electrode (A18) togenerate a high electric field, so that the electrons stored in thecharge storage regions (A17) are attracted to the first gate electrode(A18). For example, the first gate electrode (A18) is set at 12 V, andthe source (A12) the drain (A13), and the second gate electrode (A20)are set to 0 V.

Alternatively, writing of the data “0” can also be accomplished in thefollowing manner. Namely, a positive voltage is applied to the secondgate electrode (A20) to generate a high electric field, so that theelectrons stored in the charge storage regions (A17) are attracted tothe second gate electrode (A20). For example, the second gate electrode(A20) is set at 12 V, and the source (A12), the drain (A13), and thefirst gate electrode (A18) are set to 0 V. Incidentally, in thisexample, writing of the data “0” is substantially identical to erasingof the data.

Then, the read operation will be described. For example, the drainvoltage is set to be 2 V, the source voltage is set to be 0 V, and thevoltage of the second gate electrode (A20) is set to be 3.5 V, and aread pulse of 2 V is applied to the first gate electrode (A18). Thethreshold voltage differs according to whether the amount of chargesinjected into the charge storage regions (A17) is large or small.Accordingly, the drain current for storing “0” is larger than the draincurrent for storing “1”. As a result, it is possible to perform reading.Alternatively, reading can also be accomplished by interchanging thevoltage relationship between the source region and the drain region,like the following relationship: 0 V for the drain region, 2 V for thesource region, 2 V for the first gate electrode, and 3.5 V for thesecond gate electrode.

The examples of voltages for writing of the data “1” writing of the data“0”, and reading thereof described above are summarized in Table 2. Forthese operations, all the electric potentials to be applied to therespective terminals are 0 V, or they are of the same polarity.

TABLE 2 Source Drain First gate Second voltage voltage voltage gatevoltage Remarks Writing of “1” 0 V 5 V 7 V 2 V — Writing of “0” 0 V 0 V12 V  0 V Attraction (data erasure) to A18 0 V 0 V 0 V 12 V  Attractionto A20 Reading 0 V 2 V 2 V 3.5 V   —

With a conventional flash memory, writing of the data “0” isaccomplished in the following manner. Namely, a negative voltage isapplied to the gate electrode, so that electrons are allowed to tunnelthrough the insulation film and to be attracted to the substrate. Theelectric potential applied to the gate is required to be large becauseelectrons are attracted to the substrate. Further, the electricpotential is opposite in polarity to the electric potential used forwriting of the data “1”, and hence the power generation circuit becomescomplicated, incurring not only an increase in chip size, but also anincrease in cost.

In this example, writing and reading of either of the data “1” and thedata “0” can be both accomplished only by applying electric potentialswith the same polarity and almost the same magnitude. Therefore, thepower generation circuit becomes simple. As a result, it becomespossible to largely reduce the area of the peripheral circuits.

EXAMPLE 3

A description will be given to a memory device in accordance with athird example of the present invention. The basic configuration of theelements is the same as with Example 1, and the operation of eachelement is also the same. However, the memory device has features inoperation method according to the connection relationship between theelements. FIG. 5 shows an equivalent circuit diagram of the memorydevice in accordance with this example. For convenience in description,only the memory cells arranged at the center are respectively givenreference characters and numerals A70, A80, and A81, and indicated bydashed lines respectively surrounding them. Further, for the memory cellA70, the reference characters and numerals shown in FIG. 2 are given totheir respective corresponding structural elements: source A4, drain A5,gate A9, and charge storage regions A8. A71 and A76 are data lines, andconnected to the gate A9. A73 and A74 are source lines, and connected tothe source A4. A72 and A75 are word lines, and connected to the gate A9.Actually, a more larger scale memory cell array is configured, butherein, a small scale memory cell array of 3×3 is shown for explanation.

Then, a driving method of this example will be described. In thisexample, the state in which the amount of stored charges is large is setto correspond to the data “1”, while the state in which the amount ofstored charges is smaller than with the data “1” is set to correspond tothe data “0”. First, the write operation of the data will be described.The write operation to the cell (70) is performed in the followingmanner. Namely, the source line (A73) is set at 0 V. The voltage of thedata line (A71) is set, for example, at 0 V for the data “0”, or at 5 Vfor the data “1”, according to the data to be written. Whereas, avoltage pulse (ex., 12 V) is applied to the word line (A72). When thedata line voltage is set to be 0 V, few hot electrons are generated.Accordingly, the amount of charges injected into the charge storageregions is small. In contrast, when the data line voltage is set to be 5V, the amount of injected charges is large. As for other cells to bedriven by the same word line (A72) at this step, it is also possible towrite data thereto simultaneously by setting the voltage of the dataline (A76) connected thereto, for example, at 0 V for the data “0”, orat 5 V for the data “1”, according to the data to be written. Herein,writing of “0” causes no injection of charges, and hence it is equal tonot performing writing. Therefore, it is also possible to performwriting of data to only a part of the cells to be driven by the sameword line. Further, for other word lines, if the voltages thereof areset at lower voltages (ex., 0 V) than the voltage of the word lineconnected to the selected memory cell, writing will not be performed.

The erasing operation of data is performed by a single step for thecells (A80), (A70), and (A81) to be driven by the same word line (A72).Specifically, a positive voltage pulse (ex., 16 V) is applied to theword line (A72) to extract the charges stored in the charge storageregions to the word line, thereby erasing the data. At this step, thesource lines (A73) and (A74), and the data lines (A71) and (A76) are setat 0 V. Alternatively, even if 0 V is applied to either of the sourceline and the data line, and the other is allowed to be open, no problemis presented in the operation. The reason why data erasure can beachieved through application of a positive voltage to the word line,which is avoided from being carried out in a conventional flash memoryfrom the viewpoint of the reliability, is the same as in Example 1.

Further, for the memory cell in which “1” is written, by applying apositive voltage (ex., 16 V) to the word line (A72), and therebyperforming erasure, it is possible to attract the excess written chargesto the word line (A72) as described above. After erasure, if a positivevoltage is further continued to be applied to the word line, this time,electrons start to be injected from the substrate side to the chargestorage node. The speed at which charges are injected from the substrateside to the charge storage regions and the speed at which charges areextracted from the charge storage regions to the word line reachequilibrium. Accordingly, the amount of charges stored in the chargestorage regions ceases changing with time. Also in the memory cell inwhich “0” is written, similarly, the injection of charges from thesubstrate and the emission of charges to the word line reachequilibrium. Accordingly, the number of charges stored in the chargestorage regions becomes constant with time. Namely, if over-erasure isperformed on either the memory cell in which “1” is written, or thememory cell in which “0” is written, the number of charges stored in thememory cell converges to a constant number. This means thecharacteristics of respective memory cells self-converge if over-erasureis performed. Therefore, there will not occur the failure that too manycharges are extracted to render the memory cell in the normally-ONstate, which may happen for a conventional flash memory. With aconventional flash memory, the reliability cannot be ensured, and henceit is not possible to make the erasure characteristics uniform in aself-converging manner by flowing charges from the substrate to the wordline via a SiO₂ layer formed by CVD as shown in this example. It becomespossible to perform the operation of flowing charges from the substrateto the word line only after charges are stored in a plurality ofdispersed charge storage regions to ensure the reliability.

Then, data reading will be described by taking the memory cell (A70) asan example. For reading data, the source line (A73) is set at 0 V, andthe data line (A71) is precharged at a higher voltage (ex., 3 V) thanthe source line (A73) voltage. Thereafter, a read pulse with a positivevoltage (ex., 2 V) is applied to the word line (A72). At this step, whenthe data “1” is written in the memory cell (A70), and the thresholdvoltage is high, a large current does not flow, so that the data line(A71) potential does not change largely from the precharge voltage. Incontrast, when the data “0” is written, and the threshold voltage islow, a large current flows, so that the data line (A71) potential dropslargely from the precharge voltage. One end of the data line isconnected to a sense amplifier, whereby the voltage variations areamplified to read the data.

EXAMPLE 4

A memory device in accordance with a fourth example of the presentinvention will be described. The basic configuration of the elements isthe same as with Example 1, and the operation of each element is alsothe same. However, the memory device has features in the connectionrelationship between the elements. FIG. 6 shows an equivalent circuitdiagram of the memory device in accordance with this example. Forconvenience in description, only the memory cell arranged at the centeris given a reference character and numeral A110, and indicated by adashed line surrounding it. Further, for the memory cell A110, thereference characters and numerals shown in FIG. 2 are given to theirrespective corresponding structural elements: source A4, drain A5, gateA9, and charge storage regions A8. Actually, a more larger scale memorycell array is configured, but herein, a small scale memory cell array of3×3 is shown for explanation.

In this example, the source regions and the drain regions of a pluralityof memory cells are both connected by a diffusion layer to form a localsource line (A101) and local data lines (A102) and (A108). The localsource line (A101) is connected to a source line (A104) via a selecttransistor (A106). The local data lines (A102) and (A108) are connectedto a data line (A107) via a select transistor (A105). As compared withExample 3, the select transistors (A105) and (A106) become required.However, they may be provided in common to a plurality of cells to bedriven by the same local source line (A101) and the same local datalines(A102) and (A108). Therefore, the increase in cell area may besubstantially negligible. On the contrary, respective memory cells areconnected by the diffusion layer, so that it is possible to reduce thenumber of data line contacts. Accordingly, it is possible to reduce thememory cell area. In consequence, when the memory capacity is large, alarge effect is produced for cost reduction.

Then, the driving method of this example will be described. First, thewrite operation will be described. The write operation to the cell(A110) is performed in the following manner. Namely, a driving voltageis applied to gate lines (A121) and (A122) of the select transistors(A105) and (A106) to turn on the select transistors (A105) and (A106),and the source line (A104) is set at 0 V. The voltage of the data line(A107) is set, for example, at 0 V for the data “0”, or at 5 V for thedata “1”, according to the data to be written. Whereas, a high voltagepulse (ex., 12 V) is applied to a word line (A109). When the data line(A107) voltage is set to be 0 V, few hot electrons are generated.Accordingly, the amount of charges injected into the charge storageregions is small. In contrast, when the data line (A107) voltage is setto be 5 V, the amount of injected charges is large. As for other cellsto be driven by the same word line (A109) at this step, it is alsopossible to write data thereto simultaneously by setting the voltage ofthe data line (A107) connected thereto according to the data to bewritten. Herein, writing of “0” causes no injection of charges, andhence it is equal to not performing writing. Therefore, it is alsopossible to perform writing of data to only a part of the cells to bedriven by the same word line. Further, for other word lines, if thevoltages thereof are set at lower voltages (ex., 0 V) than the voltageof the word line connected to the selected memory cell, writing will notbe performed. Alternatively, writing of the data “1” to the memory cell(A110) may also be accomplished in the following manner. The data line(A107) is set at 0 V, and a positive voltage (ex., 5 V) is applied tothe source line (A104). Subsequently, a positive voltage pulse (ex., 12V) is applied to the word line (A109).

Also in the case of this example, charges are stored in a plurality ofdispersed charge storage regions to hold a high charge retentioncharacteristic. This implements the self-converging property of theerasure characteristics due to extraction of the stored charges to theword line, and flowing of the charges from the substrate to the wordline via the charge storage regions.

Then, the data reading operation will be described by taking the datareading from the memory cell (A110) as an example. The source line(A104) is set at 0 V, and a driving voltage is applied to a gate line(A122) of the select transistor (A106) to turn on the select transistor(A106). On the other hand, a driving voltage is applied to a gate line(A121) of the select transistor (A105) to turn on the select transistor(A105). Thus, the data line (A107) and the local data line (A108) areprecharged to a positive voltage (ex., 3 V). Thereafter, a read pulsewith a positive voltage (ex., 2 V) is applied to the word line (A109).At this step, when the data “1”) is written and charges are stored inthe memory cell (A110) from which the data is to be read, and thethreshold voltage is high, a large current does not flow through thememory cell (A110). Accordingly, the data line (A107) potential does notchange largely from the precharge voltage. In contrast, when the data“0” is written and no charge is stored in the memory cell (A110), thethreshold voltage is low. Accordingly, a large current flows through thememory cell (A110), so that the data line (A107) potential drops largelyfrom the precharge voltage. One end of the data line (A107) is connectedto a sense amplifier, whereby the voltage variations are amplified toread the data.

EXAMPLE 5

A memory device in accordance with a fifth example of the presentinvention will be described. The basic configuration of the elements isthe same as with Example 2, and the operation of each element is alsothe same. However, the memory device has features in the configurationof the cross section and the manufacturing method according to theconnection relationship between the elements.

FIG. 7 shows the layout diagram of the memory device in accordance withthis example. Actually, a more larger scale memory cell array isconfigured, but herein, a small scale memory cell array of 3×3 is shownfor explanation. There is an isolation region (A30) provided on a P-typesilicon substrate. Perpendicularly with respect to the isolation region(A30), a second word line (A31) made of polysilicon, corresponding tothe second gate (A20) described in FIG. 3, is disposed. In parallel tothe second word line, a first word line (A32) made of polysilicon,corresponding to the first gate (A18) described in FIG. 3, and a sourceline (A33) made of tungsten are disposed. As shown in this figure, thememory device is so configured that a group of the sequence of the firstword line (A32), the second word line (A31), the source line (A33), thesecond word line (A31), and the first word line (A32) repeatedly occurs.The respective first word lines (A32) at the ends of the neighboringgroups of the sequence are adjacent to each other. Data line contacts(A34) are disposed in the region between the adjacent first word linesexcept for the isolation region. A data line (A35) made of tungsten isdisposed so as to pass over the data line contacts (A34), and to be inparallel to the isolation region (A30).

FIG. 8 shows a cross sectional view taken on line A—A′ of FIG. 7. On aP-type silicon substrate, N-type source region (A36) and drain region(A37) are provided. On a channel (A38) connecting the source region(A36) and the drain region (A37), a 7 nm-thick insulation film (A39) isdisposed. On the insulation film (A39), the second word line (A31) madeof polysilicon is disposed. Further, on the insulation film (A39), aplurality of silicon microcrystal grains (A41) with a mean diameter of10 nm serving as charge storage regions are arranged. On the second wordline (A31), the first word line (A32) made of N-type polysilicon forcontrolling the electric potentials of the charge storage regions andthe underlying channel is disposed. Between the first word line (A32)and the silicon microcrystal grains (A41) serving as the charge storageregions, there is disposed an insulation film (A43) of a so-called ONOstructure in which a 3 nm-thick SiO₂ film, a 6 nm-thick Si₃N₄ film, anda 4 nm-thick SiO₂ film are stacked in this order from the lowest.Further, on the source region (A36), the source line (A33) formed oftungsten is formed. On the drain region (A37), a plug (data line contact(A34)) made of tungsten is formed, and connected to the data line (A35).

A manufacturing process of this example will be described. After formingthe isolation region (A30) and the triple-well structure, B (boron) ionimplantation for controlling the threshold voltage is performed on the Pwell. The substrate surface is oxidized to form a gate oxide film (A46).Subsequently, a polysilicon film and a SiO₂ film are deposited thereonfor forming the second word line (A31). By using a resist as a mask, theSiO₂ film and the polysilicon film are successively etched. In thisstep, the second word line (A31) is formed. By using the second wordline (A31) as a mask, impurities are implanted thereto to adjust theimpurity concentration under the first word line (A32). After cleaning,the substrate surface is oxidized to form a 6 nm-thick tunnel oxide film(A39). Subsequently, the silicon microcrystal grains (A41) are formed byCVD (Chemical Vapor Deposition). In a prototype, the grains were formedwith a mean grain size of 8 nm and a density of 3×10¹¹ cm⁻². Then, anONO insulation film made up of a 3 nm-thick SiO₂ film, a 5 nm-thickSi₃N₄ film, and a 3 nm-thick SiO₂ film in this order from the lowest isdeposited thereon. Thereafter, a polysilicon film for forming the firstword line (A32) and a SiO₂ film are deposited thereon. By using a resistmask, the SiO₂ film and the polysilicon film are successively etched.Then, the Si₃N₄ film and the SiO₂ film are deposited, and planarizationis performed. After planarization, a Sio₂ film is deposited again.Herein, a pattern of the source line (A33) and the data line contact(A34) is transferred to a resist. By using this resist as a mask, theSiO₂ film is etched. Even if the resist pattern of the data line contact(A34) and the source line (A33) has some misalignment, openings areproperly formed in a self-aligned manner so as to respectively exposethe drain region and the source region respectively because theunderlayer is made of Si₃N₄. Further, by etching the underlying Si₃N₄,the openings respectively exposing the drain region (A37) and the sourceregion (A36) of the substrate are formed. Herein, for ensuring thereliability of the contact, P (phosphorous) ions are implantedthereinto, followed by a heat treatment for activation. Thereafter, aSiO₂ film is deposited, and etched back to prevent a short between thedata line contacts. Then, tungsten is deposited, and planarization isperformed. After planarization, a SiO₂ film, a tungsten film, and a SiO₂film are deposited. The pattern of the data line (A35) is transferred toa resist, so that the SiO₂ film, the tungsten film, and the SiO₂ filmare etched to form the data line (A35). Subsequently, a SiO₂ film isdeposited as an interlayer film. Hereafter, the same step is repeated toperform a wiring step. In this step, the silicon microcrystal grains(A41) are left on the top and the sides of the second word line (A31).However, they will not hinder the function of the memory cell, and hencethere is no necessity to remove them.

With a conventional flash memory, the surface of polysiliconconstituting a floating gate is weakly oxidized after completion of theprocessing of the floating gate. This oxidization process improves theinsulating property of the floating gate. Accordingly, it is possible toimprove the charge retention characteristic. However, on the other hand,the gate oxide film of a peripheral circuit, or if a high-speed logiccircuit is merged therewith, the gate oxide film of the logic circuit isincreased in thickness. In consequence, the current of the transistor isreduced in amount, incurring operation delay. However, in this example,the charge storage node is formed of a plurality of silicon microcrystalgrains (A41), and hence the charge retention characteristic is high.Therefore, there is no necessity to adopt the process corresponding tothe oxidization process of the floating gate, which is suitable formerging with a high-speed logic circuit.

FIG. 9 is a circuit diagram showing the connection relationship betweenthe memory cells of the memory device in accordance with this example.Actually, a more larger scale memory cell array is configured, butherein, a small scale memory cell array of 3×3 is shown for explanation.Further, for convenience in description, three memory cells A50, A60,and A61 at the central column are indicated by their correspondingdashed lines respectively surrounding them. Further, respectivestructural elements of the memory cell A50 are given their respectivecorresponding reference characters and numerals shown in FIG. 4.

The operation of this example will be described by reference to FIG. 9.First, the write operation will be described.

Writing of data will be described by taking the memory cell (A50) as anexample. Herein, this memory cell (A50) is referred to as the selectedcell, and other memory cells are referred to as non-selected cells.Writing of data to the selected cell (A50) is performed in the followingmanner. Namely, a positive voltage pulse (ex., 6 V) is applied to a dataline (A51), a positive voltage pulse (ex., 8 V) to a first word line(A52), and a positive voltage pulse (ex., 2 V) to a second word line(A53). At this step, a source line (A54) is set at 0 V. By using such avoltage relationship, it becomes possible to generate hot electrons onthe source side, as distinct from conventional writing utilizing hotelectrons generated at the drain side. Whereas, 0 V is applied to asource line (A55) connected to non-selected cells, a first word line(A56) connected to non-selected cells, and a data line (A57) connectedto non-selected cells. To a second word line (A58) connected tonon-selected cells, a smaller positive voltage (ex., 0.5 V) than that ofthe second word line (A53) connected to the selected cell is applied. Inconsequence, it is possible to inhibit writing of data to non-selectedcells.

Data erasure is performed by a single step for the memory cellsconnected to one first word line. Below, a description will be given bytaking the memory cells (A50), (A60), and (A61) connected to the firstword line (A52) as examples. Herein, the memory cells connected to thefirst word line (A52) are referred to as the selected cells, and othermemory cells are referred to as non-selected cells. A positive voltagepulse (ex., 12 V) is applied to the first word line (A52), and apositive voltage pulse (ex., 5 V) to the second word line (A53). At thisstep, the source line (A54) is set at 0 V. In consequence, the chargesstored in the charge storage node are extracted to the first word line(A52). It is also possible that the charges stored in the charge storagenode are extracted to the second word line by reversing the voltagerelationship between the second word line and the first word line.Alternatively, it is also, of course, possible that the charges storedin the charge storage node are extracted to both the first word line andthe second word line by adjusting the voltage relationship. Whereas, 0 Vis applied to the source line (A55) connected to the non-selected cells,the first word line (A56) connected to the non-selected cells, the dataline (A57) connected to the non-selected cells, and the second word line(A58) connected to the non-selected memory cells.

Herein, the stored charges have been extracted to the first word lineand the second word line. However, it is needless to say that dataerasure may also be performed in the following manner. Namely, in thesame manner as with a conventional flash memory, 0 V or a negativevoltage is applied to the first word line and the second word line,thereby extracting the charges to the substrate.

Reading of data will be described by taking the memory cell (A50) as anexample. Herein, this memory cell (A50) is referred to as a selectedcell, and other memory cells are referred to as non-selected cell.Reading of data from the selected memory cell (A50) is performed in thefollowing manner. Namely, the data line (A51) connected to the selectedmemory cell (A50) is precharged to a positive voltage (ex., 2 V). Atthis step, the source line (A54) connected to the selected memory cell(A50) is set at 0 V. A positive voltage pulse (ex., 2 V) is applied tothe first word line (A52) connected to the selected memory cell (A50),and a positive voltage pulse (ex., 2 V) is applied to the second wordline (A53) connected to the selected memory cell (A50). Thus, the changein voltage of the data line (A51) connected to the selected memory cell(A50) is amplified by a sense amplifier. The threshold voltage of theselected memory cell (A50) differs according to whether the amount ofcharges injected into the charge storage regions of the selected memorycell (A50) is large or small. Therefore, when the data “0” is written,the threshold voltage of the memory cell is low. Accordingly, a currentflows through the selected memory cell, and hence the data line (A51)voltage decreases with time. When the data “1” is written, the thresholdvoltage of the memory cell is high. Accordingly, a current flows in asmaller amount through the selected memory cell, and hence the data line(A51) voltage shows almost no changes even when time has passed. It ispossible to perform reading by utilizing the difference.

In this example, 0 V or voltages of identical polarity are used for allof writing, erasing, and reading of data. Further, since data is writtenwith high efficiency by using source side injection by means of thesecond gate, the load on the power circuit is smaller as compared withconventional hot electron injection writing. Therefore, the peripheralcircuits, particularly, the power circuit becomes simple. This producesa significant effect on the reduction in element area when the memorycapacity is relatively small, and the area of the peripheral circuits isnot negligible as compared with the memory cell area, as in the casewhere the circuit is to be incorporated into another system, enabling alarge cost reduction.

EXAMPLE 6

FIG. 10 shows an equivalent circuit diagram of a memory device inaccordance with a sixth example of the present invention. The basicconfiguration of the elements is the same as with Example 2, and theoperation of each element is also the same. However, the memory devicehas a difference in cross sectional configuration, and has features inoperation method according to the connection relationship between theelements. FIG. 11 shows the cross sectional configuration of the memorydevice in accordance with this example.

The equivalent circuit diagram shown in FIG. 10 corresponds to FIG. 6showing the equivalent circuit diagram of the memory device based on theelements of Example 1. For convenience in description, only the memorycells arranged at the central row are given reference characters andnumerals A160 and A161, and indicated by their corresponding dashedlines respectively surrounding them. As for the memory cell A160,respective structural elements are given their respective correspondingreference characters and numerals shown in FIG. 4: source A12, drainA13, first gate A18, second gate A20, and microcrystal grains A17.Actually, a more larger scale memory cell array is configured, butherein, a small scale memory cell array of 3×3 is shown for explanation.

In this example, the source regions and the drain regions of a pluralityof memory cells are both respectively connected by a diffusion layer toform a local source line (A168) and a local data line (A165). The localsource line (A168) is connected to a source line (A163) via a selecttransistor (A162). The local data line (A165) is connected to a dataline (A164) via a select transistor (A169). As with Example 4, theselect transistors (A162) and (A169) become necessary. However, they maybe provided in common to a plurality of cells to be driven by the samelocal source line (A168) and the same local data line (A165). Therefore,the increase in cell area may be substantially negligible. On thecontrary, respective memory cells are connected by the diffusion layer,so that it is possible to reduce the number of data line contacts.Accordingly, it is possible to reduce the memory cell area. Inconsequence, when the memory capacity is large, a large effect isproduced for cost reduction.

The write operation will be described by taking writing of data to thememory cell (A160) in FIG. 10 as an example. First, the selecttransistors (A169) and (A162) are turned on, and the source line (A163)is set at 0 V. The voltages of the data line (A164) and the local dataline (A165) are set, for example, at 0 V for the data “0”, or at 5 V forthe data “1”, according to the data to be written. Whereas, a positivevoltage pulse (ex., 2 V) is applied to a second word line (A166), and apositive voltage pulse (ex., 8 V) to a first word line (A167). When thedata line (A164) voltage is set to be 0 V, few hot electrons aregenerated. Accordingly, the amount of charges injected into the chargestorage regions is small. In contrast, when the data line (A164) voltageis set to be 5 V, hot electrons are generated with high efficiency, sothat charges are injected into the charge storage regions. At this step,also for other cells to be driven by the same first word line (A167),for example, A161, it is possible to write data thereto simultaneouslyby setting the voltage of the data line (A170) connected thereto,according to the data to be written.

Data erasure is performed by a single step for a plurality of memorycells connected to one first word line. A positive voltage (ex., 15 V)is applied to the first word line, and 0 V or a positive voltage (ex.,0.5 V), which is lower than the voltage applied to the first word lime,is applied to the second word line. In consequence, the charges storedin the charge storage regions are extracted. Erasure may also beperformed in a single step for a plurality of memory cells connected tothe second word line. In this case, a positive voltage (ex., 15 V) isapplied to the second word line, and 0 V or a positive voltage (ex., 0.5V), which is lower than the voltage applied to the second word lime, isapplied to the first word line. In consequence, the charges areextracted to the second word line.

In FIG. 11, a first gate electrode (A150) is left unchanged to form thefirst word line (A167) as with the word line (A35) in FIG. 7. Similarly,second gate electrodes (A151), (A156), and (A157) form the second wordline (A166), which is wired in a direction perpendicular to the firstgate electrode (A150). Further, the drain region of one memory cell alsoserves as the source region of the adjacent memory cell. For example, adrain region (A153) of a memory cell (A152) also serves as the sourceregion of the adjacent memory cell (A154). Similarly, the source region(A155) of the memory cell (A152) also serves as the drain region of theadjacent memory cell on the left side thereof. Similarly, in theadjacent memory cell on the left side thereof, a source region-cum-drainregion is formed. A drain region (A158) is formed for the memory cell(A154) at the rightmost end. Whereas, a source region is formed for thememory cell at the leftmost end. The source regions-cum-drain regions(A153) and (A155), and the drain region and the source region onopposite ends are respectively connected by diffusion layer wiring, torun in a direction parallel to the second word line (A151). In general,with such a configuration in which source and drain regions are sharedamong a plurality of cells, and the sources and drains of a plurality ofmemory cells are connected in parallel to each other, the drain regionsof a plurality of memory cells and the source regions of theirrespective adjacent elements are required to be physically insulatedfrom each other by isolation regions. However, in this example, thememory cell isolation is achieved in the following manner. Namely, theelectric potentials of the second word lines (A151), (A156), and (A157)are controlled, so that the adjacent memory cells are electricallyisolated from each other. Thus, physical isolation is not performed, andthe isolation region is not required. In consequence, it is possible toreduce the area of the memory cell, which produces a significant effecton cost reduction.

Also in a manufacturing process of this example, the same steps as inExample 5 are performed until the step for forming silicon microcrystalgrains serving as the charge storage regions. The silicon microcrystalgrains are left on the top and the sides of the second word lines (A151)(A156), and (A157). However, also in this example, they will not hinderthe function of the memory cell, and hence there is no necessity toremove them.

The operation of this example is characterized in that writing orreading is performed on the cells to be driven by the same word line atevery one cell interval. For example, when the write or read operationis performed on the cell (A152), the second word lines (A156) and (A157)of the adjacent cells are set at a low voltage, and the silicon surfaceunder the second word line is set at a high resistance. In consequence,a short between the elements to be driven by the same first word line isprevented.

EXAMPLE 7

A seventh example of the present invention will be described. FIG. 12shows the equivalent circuit diagram of an array structure. Actually, amore larger scale memory cell array is configured, but herein, a smallscale memory cell array of 3×3 is shown for explanation. For conveniencein description, a memory cell (A202) at the central portion is indicatedby a dashed line surrounding it. FIG. 13 is a cross sectional diagramtaken along the direction perpendicular to the word line in the ovalregion indicated by a two-dot chain line of the memory cell portionadjacent in the longitudinal direction of the diagram to the memory cell(202) indicated by a dashed line surrounding it in FIG. 12. In FIG. 13,the elements denoted by a reference character and numeral (A17) aresilicon microcrystal grains which are the charge storage regions in FIG.13. This example is characterized in that memory elements are connectedin series as distinct from the foregoing examples. It is characterizedin that the configuration of series-connected elements makes theresistance high, but makes the cell area small.

Writing of data in this example will be described. In writing of data tothe memory cell (202) to be driven by a first word line (A201), a selecttransistor (A203) is turned on, and a data line (A204) is set, forexample, at 0 V for the data “0”, or at 5 V for the data “1”, accordingto the data to be written. At this step, a select transistor (A205) isturned on, and a source line (A206) is set at 0 V. Further, a first wordline (A208) and a second word line (A209) other than the first word line(A201) of the memory cell (A202) to be written and the second word line(A207) of the cell are set at a prescribed high potential (ex., 5 V forall). Thus, the channel portions under the second word line and underthe first word line are set in a low resistance state. The second wordline (A207) of the memory cell to be written is set at a lower potential(ex., 2 V) than that of other second word line (A209), so that theunderlying substrate surface is set at a relatively high resistance. Thefirst word line (A201) potential of the memory cell (A202) to be writtenis set to be higher potential (ex., 12 V) than other first word linepotential. As a result, hot electrons are generated on the substratesurface between the second word line (A207) and the first word line(A201) to be injected into the neighboring charge storage regions (A210)when the data line (A204) is set at a high voltage (ex., 5 V). Incontrast to the case where the data line (A204) potential is set to behigh potential, when the data line (A204) potential is set at a lowvoltage, few hot electrons are generated, so that few charges areinjected. The relationship between the electric potential of the firstword line (A201) of the memory cell to be written and the electricpotential of the second word line (A209) of the adjacent memory cell ismade equal to the relationship between the electric potential of thefirst word line (A201) and the electric potential of the second wordline (A207) of the memory cell (A202) to be written. As a result, hotelectrons are generated on the substrate surface between the second wordline (A209) and the first word line (A201) to be injected into theneighboring charge storage regions (A211).

Data erasure is performed on a first word line-by-a first word linebasis in the same manner as in Examples 3 to 6. A positive voltage (ex.,15 V) is applied to the first word line (A201), so that the chargesstored in a plurality of charge storage regions are extracted to thefirst word line (A201). In this step, the second word line (A207) is setat a lower voltage (ex., 0 V) than that applied to the first word line.Alternatively, erasure may also performed in the following manner.Namely, a positive voltage (ex., 15 V) is applied to the second wordline (A207), and a lower voltage (ex., 0 V) than the voltage applied tothe second word line (A207) is applied to the first word line (A201). Inconsequence, the charges are extracted to the second word line (A207).Also in this example, charges are stored in a plurality of dispersedcharge storage regions to hold a high charge storage characteristic.This implements the self-converging property of the erasurecharacteristics due to extraction of the stored charges to the firstword line or the second word line, or flowing of the charges from thesubstrate to the first word line or the second word line via the chargestorage regions.

In the read operation, the select transistor (A203) is turned on, andthe data line (A204) is precharged to a positive potential (ex., 2 V).Further, the select transistor (A205) is turned on, and the source line(A206) is set at 0 V. The first word line (A208) other than the firstword line (A201) for driving the memory cell (A202) to be read out, andthe second word line (A209) are set at a prescribed high potential (ex.,5 V for all). Further, a prescribed read voltage (ex., 3 V) is appliedto the first word line (A201). At this step, when charges are stored inthe memory cell from which data is read out, and the threshold voltageis high, a large current does not flow. In consequence, the data line(A204) potential does not show a wide range of variations from theprecharge voltage. In contrast, when no charge is stored in the memorycell from which data is to be read out, the threshold voltage is low.Accordingly, a large current flows, so that the global data line (A204)potential largely drops from the precharge voltage. One end of the dataline (A204) is connected to a sense amplifier, whereby the voltagevariations are amplified to perform reading.

In accordance with the present invention, it is possible to provide amemory element configuration whereby the required voltages are few inkind, and the voltages are low, while ensuring the reliability. By usingthis memory element, it is possible to simplify the configurations ofthe peripheral circuits of a semiconductor memory device, and to reducethe chip area. This can implement a low cost semiconductor memorydevice.

What is claimed is:
 1. A semiconductor memory element, comprising: asource region formed in a semiconductor substrate; a drain region formedin said semiconductor substrates; a channel region formed in saidsemiconductor substrate between said source region and said drainregion; an insulation film arranged on a portion of said semiconductorsubstrate corresponding to said channel region, wherein a plurality ofsilicon microcrystal grains are formed in said insulation film; and agate electrode formed over said insulation film to control electricpotential of said channel region, wherein a perimeter of each of saidsilicon microcrystal grains is covered with said insulation film so thateach of said microcrystal grains constitutes an independent and isolatedcharge storage region such that said plurality of said charge storageregions are electrically disconnected, and wherein an electric potentialto be applied to said gate electrode upon writing of data and anelectric potential to be applied to said gate electrode upon erasing ofdata have the same polarity.
 2. A semiconductor memory element accordingto claim 1, wherein said plurality of microcrystal grains have a meansize of approximately 10 nm.
 3. A semiconductor memory element accordingto claim 1, wherein said plurality of charge storage regions storecharges from the same bit of data.
 4. A semiconductor memory elementaccording to claim 1, wherein said insulation film is comprised ofsilicon oxide.
 5. A semiconductor memory element according to claim 1,wherein said gate electrode is comprised of a metal or a semiconductor.6. A semiconductor memory element, comprising: a source region formed ina semiconductor substrate; a drain region formed in said semiconductorsubstrate; a channel region formed in said semiconductor substratebetween said source region and said drain region; insulation filmarranged on a portion of said semiconductor substrate corresponding tosaid channel region; and a gate electrode formed over said insulationfilm to control electrical potential of said channel region, whereinsaid insulation film includes means for dispersing charges correspondingto one bit of data written into said semiconductor element into aplurality of independent isolated charge storage regions, and wherein anelectric potential to be applied to said gate electrode upon writing ofdata and an electric potential to be applied to said gate electrode uponerasing of data have the same polarity.
 7. A semiconductor memoryelement according to claim 6, wherein said insulation film is comprisedof silicon oxide.
 8. A semiconductor memory element according to claim6, wherein said gate electrode is comprised of a metal or asemiconductor.
 9. A semiconductor memory element according to claim 6,wherein said means for dispersing charges includes a plurality ofsilicon microcrystal grains formed in said insulation film which areelectrically disconnected from one another.
 10. A semiconductor memoryelement according to claim 9, wherein said plurality of microcrystalgrains have a mean size of approximately 10 nm.
 11. A semiconductormemory element, comprising: a source region formed in a semiconductorsubstrate; a drain region formed in said semiconductor substrate; achannel region formed in said semiconductor substrate between saidsource region and said drain region; an insulation film arranged on aportion of said semiconductor substrate corresponding to said channelregion; and a gate electrode formed over said insulation film to controlelectrical potential of said channel region, wherein said insulationfilm includes means for dispersing charges corresponding to one bit ofdata written into said semiconductor element into a plurality ofindependent isolated charge storage regions, and means for permittingwriting of data and erasing of data to and from said independentisolated charge storage regions by applying a first electric potentialto said gate electrode for writing of data and applying a secondelectric potential to said gate electrode for erasing of data, whereinsaid first and second electric potentials each have the same polarity.12. A semiconductor memory element according to claim 11, wherein saidmeans for dispersing changes and permitting writing and erasing of dataincludes a plurality of silicon microcrystal grains formed in saidinsulation film which are electrically disconnected from one another,such that said insulation film separates said silicon microcrystalgrains from each other and from both the substrate and the gateelectrode.
 13. A semiconductor memory element according to claim 12,wherein said plurality of microcrystal grains have a mean size ofapproximately 10 nm.